1. Field of the Invention
The present invention relates to an objective actuator which is, for example, used in an optical recording and playback device adapted to focus a light beam from a light source such as a semiconductor laser onto an information recording medium to record and reproduce information.
2. Brief Description of the Prior Art
The architecture of the conventional objective actuator of this type is illustrated in FIGS. 8 and 9. Thus, a base 1 comprising a magnetic metal plate to be fixed between a light source and an information recording medium in an optical information recording and playback system is provided with a through-aperture 2 for passage of the light beam from a light source. The base 1 is further provided with an installation hole 4 for rigidly supporting a shaft 3 in a position adjacent to said aperture 2 and a rubbery spring 5 disposed on one side of said installation hole which is opposite to the side on which said aperture 2 is situated. Disposed in juxtaposition on a straight line perpendicular to the line interconnecting said aperture 2, installation hole 4 and rubbery spring 5 are a pair of arcuate magnet holders 7a and 7b which are designed to support projecting arcuate magnets 6a and 6b, respectively. Arcuate magnetic path members 8a and 8b are disposed externally of said magnet holders 7a and 7b at a predetermined spacing therefrom. A bearing 9, which is freely rotatable and vertically movable, is fitted over shaft 3. An objective holder 10 made of polyphenylene sulfide resin, epoxy resin or the like material is fitted and secured in position around bearing 9. An objective 11 is fitted into beam incidence aperture 19 formed in objective holder 10 in a position corresponding to the aperture 2 of base 1. A line passing through the center of objective 11 and apertures 19, 2 and 17 comprises the light axis of the optics, or the optic axis. A balancer 13 having a retaining pin 12 engageable with rubbery spring 5 is assembled into objective holder 10. Attached to the periphery of objective holder 10 is a ring-shaped focusing control coil 14, the outer periphery of which, in turn, is provided with four arcuate tracking control coils 15. These two kinds of coils are disposed in a space between the magnets 6a, 6b on the one hand and the magnetic path members 8a, 8b on the other hand. All the above parts, as assembled, are covered with a cover 16, which is formed with relief windows 17 and 18 in positions corresponding to the objective 11, bearing 9 and balancer 13.
In the above arrangement, defocus of the light beam with respect to the surface of the information recording medium and off-track of the light beam with respect to a track groove on the information recording medium, is detected. In accordance with the respective detection signals, the focusing control coil 14 and tracking control coils 15 are energized to cause the magnets 6a and 6b and magnetic path members 8a and 8b to form a magnetic field and, utilizing the resulting magnetic flux, drive the object holder 10 up and down and rotate it to control the relative position of the objective 11.
However, in the above arrangement, since the movable segment comprised of objective 11, objective holder 10, bearing 9, focusing control coil 14, tracking control coils 15 and balancer 13 is secured in position with an adhesive or cement, the movable segment has low high-order resonance frequencies and high resonance peak values at these high-order resonance frequencies both in the focusing direction parallel to the light axis of the optics and in the tracking direction perpendicular to the light axis of said optics. This makes it difficult to allow the beam spot from the objective following the plane deflection and radial offset of the information recording medium, that is to say, the stability of servo performance is poor.